Director Kevin Macdonald (The Last King of Scotland) is taking charge of the adaptation of Meg Rosoff's novel.
Ronan will star as willful 15-year-old Daisy, who is sent from Manhattan to stay with her aunt and three male cousins who she has never met, reports Variety.
After an attack on London the four young people are left to fend for themselves on an isolated farm, without power, in the middle of a war-torn country.
Jeremy Brock (The Eagle, The Last King of Scotland) and Tony Grisoni (The Red Riding Trilogy) are adapting the novel.
Ronan starred in Hanna earlier this year and is due to appear in the forthcoming movies Violet and Daisy, Byzantium and The Host.
How I Live Now is currently in pre-production and is scheduled for release in UK cinemas during 2013.
Watch Saoirse Ronan in a trailer for Hanna below: